1. Technical Field
This invention relates to the technology of foundry sand reclamation and more particularly to a more economic, nimble and robust gravity flow foundry sand reclamation system.
2. Discussion of the Prior Art
Calcining is an effective thermal process to reclaim resin bonded foundry sand. It is a thermal process that raises the temperature of the sand grains to a point where the spent organic resin coating is oxidized, leaving clean undamaged sand grains that are equal to new sand and can be reused in the foundry. Most sand reclaimers currently used in large foundries throughout the world are fluid bed type of calciners. This type of calciner heats large volumes of air to a calcining temperature and then passes such air through a bed of sand that is to be reclaimed. This procedure provides the temperature required to combust organic material and requires an extremely large volume of air for combustion as well as to fluidize the sand bed. Usually the resin coated sand is metered into the fluidized bed reclaimer from one end or side of the calciner and migrates through the reclaimer to flow out the reclaimer at the other end or side. Typically the air of a fluidized bed calciner is heated using natural gas or electric energy. The extremely large volume of heated air, containing the gaseous products of the resin, as well as particulate sand dust, must be treated to remove such contaminates before emissions can be discharged to the environment. This treatment may further include incineration, cooling and filtering. With the unusually large volume of gaseous emissions, the task can be expensive and formidable. Since these fluid bed type calciners are typically large refractory lined vessels with pressure fans and burner systems designed to move and heat large volumes of sand and fluidizing air, they must be operated continuously with a full load of fluidized sand and cannot be operated to empty or restarted very quickly. They consume large amounts of energy and require sizable emission control equipment due to the high volume of air used.
Recent attempts to substitute a gravity flow calcining system for a fluid bed type of system, to achieve better system control and flexibility, have not be entirely successful. One such gravity flow system is disclosed in related U.S. Pat. Nos. 5,110,288 and 5,165,888. The system of these patents requires a furnace chamber with combustion zones separated elevationally through the full height of the furnace chamber; the zones have radially directed burner units to turbulently and violently engage the sand that is quickly falling therethrough to attempt to combust the resin binder coating the sand particles. A few angled baffles extend across the furnace to attempt to slow down the descent of sand from one combustion zone to another. Unfortunately heat transfer to the resin coating on the sand grains must be entirely by convection which is not entirely effective in combusting all of the resin during the flow of sand through the combustion zones. It is necessary for the system to require a secondary chamber to carry out further combustion to complete the reclamation that was not fully carried out by the furnace. This is not energy efficient and results in undesirable production of ultra fine dust particles due to the turbulent collisions of the sand grains in the combustion zones that degrade the quality of the reclaimed sand.